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Water Hardness. Calculating Calcium.

  1. Jun 24, 2015 #1
    1. The problem statement, all variables and given/known data

    I have been asked to determine the concentration of calcium ions, in a 100 mL sample of water.
    Eriochrome black T Indicator was used. Magnesium was removed through a precipitate. The water sample changes colour to red. pH 10 buffer solution was also used.
    the 3 test amounts of 0.01 mol EDTA solution were
    11.07mL, 11.57mL and 10.57mL giving an average titre of 11.07mL

    2. Relevant equations
    moles(EDTA) = concentration (mol) x volume (L)

    Calculate moles of Ca2+ present in the 100.0 mL water sample:
    moles Ca2+ = moles EDTA

    Calculate concentration (molarity) of calcium ions in the water:
    molarity(Ca2+) = moles ÷ volume (L)

    This is as far as I could figure out
    3. The attempt at a solution
    moles(EDTA) = concentration (mol) x volume (L)
    = 0.01 x 11.07 x 10-2 = 11.07 x 10-4 mol

    Calculate moles of Ca2+ present in the 100.0 mL water sample:
    moles Ca2+ = moles EDTA = 11.07 x 10-4 mol

    Calculate concentration (molarity) of calcium ions in the water:
    molarity(Ca2+) = moles ÷ volume (L) = 11.07 x 10-4 ÷ 100.0 x 10-3
     
  2. jcsd
  3. Jun 24, 2015 #2

    Borek

    User Avatar

    Staff: Mentor

    In general you are on the right track, but your math is incorrect here:

    10-2 is wrong (as long as it is intended to be a conversion factor between mL and L, I don't see what else you could use here).
     
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